Assuming its the same size as all other Phantom cards, its 2½ slots. So you still have to have 3 slots between your cards to use these in SLI, but the cards will have a little more room to breathe than a full 3 slotter.

Yeah, it's 2½ slots.
The phantoms are awesome looking cards and I'd definately love one, but in my books it loses to some coolers like msi's tf or gigabyte's x3 because it's 2½ slots instead of 2. If that doesn't bother you, it's definately a great buy. It's cool, silent and great looking card.